Correction device for electron lenses



y 1949. J. HILLIER 4 2,469,165

CORRECTION DEVICE FOR ELECTRON LENSES Filed Oct. 29, 1946 NE 776' M/TER/A'L MiG/V5 77 4775/7 Summer James- Hillier cam Patented May 3, 1949 CORRECTION DEQZCE FOR ELECTRON LENSES James Hillier, Cranbury, N. .I., assignor to Radio Corporation of America, a corporation of Dela- Ware Application October 29, 1946, Serial No. 706,343

Claims.

This invention relates generally to electron optical apparatus and more particularly to a correction device for electromagnetic electron lenses for correcting for assymrnetries in the magnetic structure of such lenses.

In conventional electron microscope electromagnetic objective or projection lenses, the pole pieceassemhly comprises a symmetrical cylindrical or conical unit supported on the axis of the activating electromagnetic winding. In this an rangement the portion of the magnetic circuit providing the lens action comprises a pair of pole pieces havin extremely small axial apertures and having oppositely disposed faces separated by a relatively short air gap. The air gap dimensions are determined by a non-magnetic separator interposed between the outer surface of the cylindrical pole pieces, whereby the pole piece assembly comprises a cylindrical unit which may be readily removed from the axial region of the lens winding and its magnetic structure. The pole piece assembly closely engages the ends of the magnetic structure surrounding the top and bottom portions of the winding assembly.

Since the pole piece assembly must be readily removable for cleaning and adjustment, it is essential that it be relatively loosely fitted into the lens winding magnetic structure. The central aperture in the magnetic structure and the pole piece assembly are machined as accurately as possible in order that the pole piece assembly always may be replaced in substantially the same manner after removal. However, such an arrangement necessarily requires extremely narrow clearance air gaps between the magnetic structure and the adjacent pole pieces. It is essential that these gaps be as small as possible and that they be substantially uniform at all points around the pole pieces. Practical manufacturing considerations and variations in the magnetic characteristics of the magnetic structure and the magnetic pole piece introduce assymmetries in the lens which are particularly objectionable at high lens activation strengths. The assyrnmetries in the lens are caused by the variations in magnetic field density in the various circumferential portions of the pole piece assembly.

The instant invention comprises a thin, soft iron, cylindrical magnetic shield extendin into one or both of the pole pieces, and isolated magnetically from the pole piece, to provide a field free space for the electron beam entering or leaving the lens. Thus, the electron beam is substantially unaffected by the lens field until it reaches a region immediately adjacent to the central lens air gap in which the lens field provides the desired electron focusing action. Since the electrons pass through a substantially field free region upon entering and/ or leaving the lens, assymmetries in the magnetic field distribution in the outer portions of the pole piece structure have substantially no effect upon the focusing action or displacement of the electron beam, and higher resolution images may be obtained than have been possible with lens structures heretofore employed.

Among the objects of the invention are to provide an improved method of and means for correcting for magnetic assymmetries in electromagnetic electron lenses. Another object is to provide an improved electromagnetic electron lens correcting device. A further object is to provide an improved method of and means for efiectively isolating an electron beam entering and/ or leaving an electron lens from extraneous and assymmetrical magnetic fields existing in the regions adjacent to the lens plane. An additional object is to provide an improved electromagnetic lens field aligning or correcting device comprising a cylindrical shield of soft magnetic material enclosed Within, but magnetically isolated from, the large entrance or exit portions of the magnetic lens pole pieces. A still further object is to provide an improved method of and means for introducing an electron beam into a magnetic electron lens through a substantially field free region to correct for magnetic assymmetries in the lens magnetic structure.

invention will be described in greater tie-- tail by reference to the accompanying drawing of which the single figure thereof is a cross-sectional elevational view of a conventional electromagnetic electron lens including the invention.

Referring to the drawing, a. preferred embodiment of the invention is illustrated in combination with an electromagnetic electron lens including a solenoid winding I wound upon a nonmagnetic core 3 composed, for example, of brass. A. pair of annular members 5 and l, of soft magnetic material, and a circumferential member 9, also or soft magnetic material, surround the winding i. The central aperture Il'extending through the ncn-magnetic core 3 and the center portions of the annular magnetic members 5 and 1 is machined to fit a pole piece assembly comprising a. pair of cylindrical pole pieces I 3, I5 having a short gap ll therebetween. The spacing of the pole pieces l3 and i5 is established by a non-magnetic spacer it threaded into both of the pole pieces.

The length of the air gap ll may be adjusted by substituting difierent sized non-magnetic spacing elements. ihe opposing faces of the pole pieces l3, 15 include small central lens apertures ll, 23, respectively, for passage of the electron beam through the lens pole piece assembly. The central apertures of the pole pieces thence are widened out to localize as far as possible the lens action of the opposing pole piece faces adjacent to the air gap IT.

The loose fit between the central openings in the annular magnetic members 5 and i and the outer walls of the pole piece assembly provides a very narrow air gap 25, the non-uniformity of which causes magnetic assymmetry in the portions of the pole piece walls remote from the air gap ll. Such magnetic assymmetries also are caused by non-uniformity in the magnetic characteristics of the pole pieces l3, l5 and of the annular magnetic elements 5, 1. This assymmetry in the magnetic structure of the lens causes distortion and/or deflection of the path of the electron beam entering or leaving the lens which confuses the electron beam alignment in the system. While great care usually is employed in machining the lens pole piece assembly and the associated magnetic structure, additional correction for such lens assymmetries is necessary to provide a lens of high magnification and resolution.

According to the instant invention, the effects of such magnetic assymmetries may be substantially corrected by the insertion, within the large central aperture of either one, or both, of the lens pole pieces l3, E5, of a thin, hollow, cylin- I drical tube 2'5, of soft magnetic material, extending substantially entirely throughout the length of the large central lens aperture and isolated magnetically from the adjacent pole piece walls by means of non-magnetic spacers 29, 3!. It should be understood that such a cylindrical shield may be employed in the large central aperture of only one of the pole pieces, if desired, and as illustrated in the drawing The magnetic cylindrical shield 21, being magnetically isolated from the adjacent lens pole piece element i5, effectively provides a field free space between the outer end of the lens and the region thereof immediately adjacent to the central lens plane in the air gap ll. Since an electron beam projected through the lens is effectively isolated from the lens magnetic field during its passage through the shielding cylinder 21, the magnetic assymmetries in the lens magnetic structure and pole piece elements is greatly minimized. If the cylindrical shield 2'1 is electrically connected to the pole piece walls and to the lens magnetic structure, although magnetically isolated therefrom, the grounding effect thereof tends to prevent charge accumulation on the shield which might otherwise afi'ect the path of the electron beam projected therethrough. It should be understood that the opening 33 through the cylindrical shield 21 must be sufficiently large to permit the passage of all desired electron rays entering or leaving the lens assembly, depending upon whether the shielding element is utilized in the upper or lower pole piece elements.

lhus the invention described comprises an improved method of and means for effectively correcting for magnetic and mechanical assymmetries in an electromagnetic lens assembly by including an axially disposed, cylindrical, magnetic shield Within the lens structure, said shield being efiectively magnetically isolated from the adjacent pole piece elements.

I claim as my invention:

1. Apparatus for compensating for mechanical and electromagnetic assymmetries between the electromagnetic structure and pole pieces in an electron lens employed for focusing an electron beam, including an electromagnetic field aligning element concentric with said beam in the region substantially entirely within said structure and coaxial with one of said pole pieces.

2. Apparatus according to claim 1 including non-magnetic means for supporting said element within said pole piece.

3. Apparatus for compensating for mechanical and electromagnetic assymmetries between the electromagnetic structure and pole pieces in an electron lens employed for focusing an electron beam, including a hollow cylindrical electromagnetic field aligning element concentric with said beam in the region substantially entirely within said structure and coaxial with one of said pole pieces.

4. Apparatus according to claim 3 including an annular non-magnetic member for supporting said element within one of said pole pieces.

5. Apparatus according to claim 3 wherein at least one of said pole pieces includes a cylindrical portion having a relatively enlarged central aperture, and non-magnetic means for supporting said element within said enlarged aperture of said pole piece.

JAMES HILLIER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,234,281 Ruska Mar. 11, 1941 2,294,123 Miller et al Aug. 25, 1942 2,330,628 Ruska Sept. 28, 1943 

